Support multiple named profiles for `claude auth`

Summary

The claude auth command currently supports only a single authenticated account at a time. Switching between accounts (e.g. a personal claude.ai account and a work enterprise account) requires claude auth logout followed by claude auth login, which interrupts the workflow and does not preserve the previous session's credentials.

Proposed feature

Add named profile support to the CLI, similar to how aws configure --profile or gh auth switch works:

claude auth login --profile personal
claude auth login --profile elastic

claude --profile personal   # open a session with the personal account
claude --profile elastic    # open a session with the work account

A default profile could be set with:

claude auth default personal

Use case

Engineers who have both a personal subscription and an employer-provisioned enterprise account need a way to choose which account to bill and which context to use without a full logout/login cycle each time they open a new session.